1. Field
This invention relates to wedge tool for welding fine wires in the semiconductor chip industry. More particularly, the present invention relates to a wire bonding method and apparatus that enables welding wires to bonding pads positioned at an ultra-fine pitch.
2. Related Arts
Wire bonding is used in a semiconductor industry to electrically connect a chip to the circuit board. A plurality of bonding pads are located in a pattern on the top surface of the board, with the chip mounted in the center of the pattern of bonding pads. In standard chips, i.e., non-flip chip, the top surface of the chip facing away from the top surface of the substrate has a plurality of contact pads. Fine wires (which may be aluminum or gold wires) are connected between the contact pads on the top surface of the chip and the contact pads on the top surface of the board.
In general, two technologies have been developed for performing the welding of the fine wires: ball bonding and wedge bonding. In ball bonding, sometimes referred to also as capillary bonding, the connecting wires are supplied and bonded to the chip and to the substrate through a capillary tunnel traversing the entire length of the ball bonding tool. In this respect, the terms “bonding tool” “ball bonding tool” and “wedge bonding tool” refer to a tip which is installed on a bonder machine and which is used to feed and bond the wire to the pads. The ball bonding method has dominated the interconnect market because of its high speed and capabilities. However, as the need for much denser interconnects is increasing (the ITRS roadmap predicts that we are heading towards 20 μm pitch), ball bonding is reaching its limits. On the other hand, since wedge bonding can produce a smaller full strength weld than ball bonding, wedge bonding has the potential to dominate the market in ultra-fine-pitch devices.
Wire bonding is a welding process accomplished by deforming the wire and the substrate together, forming them into an alloy of the two constituents. Ultrasonic energy enhances the process by lowering the flow stress and allowing easy slip mechanisms for dislocation movement (deformation occurs by the movement of dislocations). Wedge bonding, because it directly deforms the wire without first forming a ball, is capable of producing a weld with less deformation than ball bonding. High quality welds can be produced with bond width that is only 20-25% larger than the wire diameter. This size is significantly smaller than the minimum size that a ball bonder can produce for the same wire diameter.
FIG. 1 shows the common bonding processes and compares their deformation as a function of initial wire diameter for an optimized process. Notably, the deformation caused by ball bonding requires much larger pitch than that is required by wedge bonding. Therefore, in order to achieve finer pitch capabilities, the wire diameter must be reduced—regardless of the type of bonder used. For ball bonders, reducing the wire diameter allows ball formation and subsequent deformation with a smaller final bond diameter. However, as wire diameter is decreased, conductivity and strength of the bond are also reduced. Many new devices require more current flow and the smaller diameter wire required by finer pitch devices has a negative impact on performance and reliability of the device. Wedge bonding has an advantage, because it can achieve equivalent pitch with a larger diameter wire, or finer pitch with the same diameter wire as ball bonding.
In order to develop a wedge bonder that would enable ultra-fine pitch bonding, two limitations need to be addressed. First, the ability to locate the wedge tip exactly on the desired location, including the ability to perform complicated processes. Second, the wedge tool dimensions must be reduced without affecting its strength, so as to allow working in narrow pitch and crowded wires environment.
Wedge bonder manufacturers have improved the bonders' capabilities, such that they are much faster than previous generation machines and now are capable of bonding speed of above 6 wires per second. New constant loop height and constant loop length algorithms achieve optimum control of electrical properties (impedance, capacitance). Low loop capability is superior because there is no ball adding to the bond height and the low take off angle of the wire naturally tends toward lower height. New looping motion algorithms provide the lowest loops achievable. In eight-die stacks, wedge bonds have been shown to achieve a 32% decrease in cross section and a 20% decrease in total stack height compared to standard ball bonded devices. This provides an inexpensive alternative to through silicon vias (TSVs) in 3-D packaging. Also, bond placement accuracy has improved. New machines are capable of 1 μm bond placement repeatability at 3σ.